The present invention relates to the manufacture of profiled products by extrusion of expandable synthetic resinous material, and more especially pertains to the production of profiled products having non-foamed surfaces by the extrusion of styrene polymer compositions having a particular formulation.
Relatively recently processes have been developed for the production of profiled products by extrusion of certain expandable synthetic resinous materials, particularly polyvinyl chloride, polyamides, polycarbonates, polyurethanes and the like. These processes are characterized in that the synthetic resinous material is extruded through an extrusion die in a substantially unexpanded state, the extrusion die having the shape of the profile desired, and thereafter allowing the resinous material to expand in a shaping chamber which has the same profile as the extrusion die orifice and in addition is of the final desired cross-sectional area of the ultimate product to be obtained. As the resinous material expands against the walls of the shaping chamber, the outer surfaces of the profiled product thus formed take on a smooth, non-foamed character whereas the interior portion of the article cross-section remains as a light weight, porous material. Alternatively, there may also be a hollow internal cavity positioned inside the extruded aricle, and the process may be carried out such that the surface layer surrounding such an internal cavity is also of a non-foamed character.
As is the case with profile extrusion methods utilizing any of the above-mentioned polymeric materials, and particularly when effort is made to employ styrene polymer compositions in such profile extrusion, many problems arise in achieving an extruded product having a good, smooth, relatively hard and tough non-foamed surface and in obtaining such surface without loss of desired density of the final profile. Although a multitude of known expandable styrene polymer compositions, i.e., styrene polymer plus blowing agent and optionally a nucleating agent, are known for expanding polystyrene in sheet extrusion and in other applications, it has been found that the majority of such known compositions capable of producing expanded polystyrene do not provide the desired results in terms of density and surface characteristics when utilized in profile extrusion. Moreover, because it is necessary to initially extrude the polymeric material in a substantially unexpanded state, when utilizing many of the known expandable styrene polymer compositions it is necessary to introduce the blowing agent into the styrene polymer material at a point well advanced through the extrusion equipment, i.e., only shortly before the material passes through the extrusion orifice. This, of course, is a disadvantageous feature which adds an additional requirement to conventional extrusion processes and requires a greater outlay for equipment. Even in those expandable styrene polymer compositions which permit the blowing agent to be pre-incorporated in the polymeric material, e.g., see for example, application Ser. No. 380,009 filed concurrently herewith, problems arise because of susceptibility to temperature degradation and poor shelf life.